The move - based on a revenue-sharing model - gives JWT an effective operation in all three of the primary cities in the country, including Shanghai and Beijing.
JWT Northeast Asia area director and Greater China CEO Tom Doctoroff said the agency's current office in Guangzhou is only staffed by two people.
"For all intents and purposes, we haven't had an agency in the city for the past few years at least."
New Sun is headed by a husband and wife team - James Wong, a former director of strategic planning at Ogilvy & Mather; and Li Ming, ranked as one of China's top creatives.
It is staffed by 115 people, who work on a range of accounts such as Brilliance Auto, Li Bai (detergent), GSL (electronic dictionary) and Olympia Garden (real estate).
Doctoroff said: "We have signed a strategic alliance and talks to forge a deeper financial arrangement is on-going."
However, he stressed that there would be no restructuring of the local agency. "They know what they are doing and they are doing it well. Their model isn't our model, but it is a powerful long-term proposition."
they get an association with an international brand without having to sacrifice their way of doing business. What that will mean is that we will provide a number of resources including participating in their pitches and arranging training."